This invention concerns a catalyst composition and its preparation and use. In particular, this invention concerns a catalyst composition having a finely divided silica component and a hydrogenating component comprising copper and zinc oxide. The composition is an extremely active hydrogenation catalyst useful in the hydrogenation of esters to alcohols, particularly the hydrogenation of esters of glycolic acid to ethylene glycol.
The hydrogenation of esters to alcohols is well known. In general, the hydrogenation of an ester sometimes referred to as a reduction of the ester, is carried out using either a chemical reducing agent or a hydrogenation catalyst. The chemical reducing agents, such as lithium aluminum hydride, react through the hydride ion and are usually used in ether solution. However, for large scale laboratory or industrial preparations, the catalytic reduction of esters, under hydrogen pressure and increased temperature, is preferred. The most widely used catalyst is the so-called copper chromite catalyst (H. Adkins in Vol. VIII of "Organic Reactions", R. Adams, Ed., pp. 1-27, 1954).
A large amount of research effort has been expended in attempts to improve the catalytic hydrogenation process. In particular, various methods have been proposed for improving the catalyst composition to provide a more active hydrogenation catalyst. For instance, copper catalysts have been modified by the addition of other Group IIA or VIIA metals or their oxides, by the addition of activating substances, or by the use of a catalyst support.
U.S. Pat. No. 2,091,800 to Adkins et al discloses the use of a copper chromite/barium catalyst. Similarly, U.S. Pat. No. 2,093,159 to Schmidt discloses the use and combination of various catalytic metals and activating substances. According to the disclosure of U.S. Pat. No. 2,093,159:
"Suitable catalytic substances are for example copper, nickel, silver, zinc, cadmium, lead, or cobalt, or mixtures thereof and they may be prepared from their salts, oxides or other compounds prior to or after their incorporation with activating substances. The activating substances may be chosen from compounds of the metals giving acids with oxygen, such as chromium, molybdenum, tungsten, uranium, manganese, vanadium, or titanium or mixtures thereof as well as from compounds of the alkali, alkali earth or rare earth metals."
U.S. Pat. Nos. 2,109,844; 2,110,843; 2,118,007, 2,121,367; 2,782,243; 3,173,959; 3,267,157 and 2,285,448 disclose other combinations of hydrogenating metal catalysts and activators.
Although a wide variety of supports and modifiers have been used with copper hydrogenation catalysts, a limited number of types have found commercial acceptance. These are (1) copper chromite, with modification such as addition of BaO or MnO.sub.2, (2) Cu-magnesia and (3) copper or copper oxide with zinc oxide. Copper-zinc oxide catalysts are commonly used for formation of alcohols by hydrogenation of esters or of aldehydes obtained by the "oxo" process. They are sometimes used for hydrogenation of the double bonds of fats. A typical example is contained in U.S. Pat. No. 2,305,104, where a Cu-Zn-Cr oxide catalyst is used for vapor phase hydrogenation of methyl glycolate (methyl hydroxyacetate) to ethylene glycol. They may also be used for methanol synthesis or for the reaction of carbon monoxide with steam to form hydrogen. For these reactions of carbon monoxide, the copper-zinc oxide catalysts are often modified with alumina or chromia.
Besides the use of activating substances and metal catalyst combinations, the use of an inert carrier or support is a common method used to improve catalyst performance. The support typically provides a large surface area on which the active component is spread out to give a larger surface area of active component, although it may serve other purposes as described by W. B. Innes in Chapter 6, Vol. 1 of "Catalysis", Paul H. Emmett, Editor, Reinhold, 1954.
Generally, large amounts of support are used relative to active component(s). Typically, supported catalysts will contain 40 to over 99 weight percent support.
Silica gel is a commonly used catalytic support. Its polularity is principally due to the presence of high-surface area silica having an abundance of surface hydroxyl groups. French Pat. No. 2,256,780 opened Aug. 1, 1975 describes a process for preparing a cupric catalyst with a silica-containing support. The process is characterized by the introduction of silica gel into a support consisting of hydrated copper (II) oxide applied to kieselguhr. U.S. Pat. No. 3,515,678 to Koritala also describes supported copper hydrogenation catalysts using either micronized silica or molecular sieve zeolites.
Silica-containing materials have also been used to promote catalytic activity by means other than supporting the active metal. For instance, British Pat. No. 759,426 describes promoted cobalt oxide catalysts of increased activity prepared by introducing colloidal silica or colloidal titanium into a purified aqueous cobalt nitrate solution. Similarly, U.S. Pat. No 3,652,458 to Gobron et al describes hydrogenation catalysts comprising nickel or copper deposited on a highly pure silica. The catalyst is formed by depositing the metal catalyst on silica in colloidal suspension.